RFR 8047737 Move array component mirror to instance of java/lang/Class

[Peter Levart]

On 07/02/2014 02:38 PM, Peter Levart wrote:
> On 07/02/2014 02:22 PM, Peter Levart wrote:
>> On 07/02/2014 08:26 AM, Mandy Chung wrote:
>>>
>>> On 6/30/2014 9:51 PM, Christian Thalinger wrote:
>>>> On Jun 30, 2014, at 5:50 PM, Coleen Phillimore 
>>>> <coleen.phillimore at oracle.com> wrote:
>>>>
>>>>
>>>> On 6/30/14, 3:50 PM, Christian Thalinger wrote:
>>>>>>       private Class(ClassLoader loader) {
>>>>>>           // Initialize final field for classLoader. The 
>>>>>> initialization value of non-null
>>>>>>           // prevents future JIT optimizations from assuming this 
>>>>>> final field is null.
>>>>>>           classLoader = loader;
>>>>>> +        componentType = null;
>>>>>>       }
>>>>>>
>>>>>> Are we worried about the same optimization?
>>>>> Hi,  I've decided to make them consistent and add another 
>>>>> parameter to the Class constructor.
>>>>>
>>>>> http://cr.openjdk.java.net/~coleenp/8047737_jdk_2/
>>>
>>> The jdk change looks okay while I am beginning to think whether we 
>>> really want to keep expanding this constructor to deal with this 
>>> future JIT optimization (you will be moving more fields out from the 
>>> VM to java.lang.Class).
>>>
>>> There are places in JDK initializing the final fields to null while 
>>> the final field value is overridden via native/VM - e.g. System.in, 
>>> System.out, etc.  I would prefer reverting the classLoader 
>>> constructor change to expanding the constructor for any new field 
>>> being added.  Handle it (and other places in JDK) when such JIT 
>>> optimization comes.
>>>
>>> Mandy
>>>
>>
>> What about:
>>
>>
>>     private Class() {
>>         classLoader = none();
>>         componentType = none();
>>         ...
>>     }
>>
>>     private <T> T none() { throw new Error(); }
>>
>>
>> I think this should be resistant to future optimizations.
>
> And you could even remove the special-casing in 
> AccessibleObject.setAccessible0() then.
>
> Regards, Peter

I take it back. Such java.lang.Class instance would still be constructed 
and GC will see it.

>
>>
>> Regards, Peter
>>
>